The invention relates to a method for depositing in particular crystalline layers on in particular likewise crystalline substrates, in which at least two process gases are introduced into a process chamber of a reactor separately from one another, the first process gas flowing through a central line with a central outlet opening and the second process gas flowing through a line which is peripheral with respect to the central line and has a peripheral outlet opening, the second process gas flowing through a plurality of feed lines into a mixing chamber and through further means, which influence the gas stream, for homogenizing the radial flow profile of the process gas emerging from the peripheral outlet opening.
The invention also relates to a gas-admission element on a device for depositing in particular crystalline layers on in particular likewise crystalline substrates, by means of which at least two process gases can be introduced into a process chamber of a reactor separately from one another, having a central line with a central outlet opening for the first process gas, and having a line, which is peripheral with respect to the central line and has a peripheral outlet opening for the second process gas, which peripheral line has means, which influence the gas stream between one or more feed lines opening out into a mixing chamber and the peripheral outlet opening, for homogenizing the radial flow profile of the process gas emerging from the peripheral outlet opening.
A device of this type is already known, for example from U.S. Pat. No. 6,080,642. This document discloses a gas-admission element with two gas feed lines, each for one process gas, which are fed to the process chamber separately from one another. There, arsine or phosphine is guided through the gas-admission element through a central line. The central line ends at the end side of the substantially rotationally-symmetrically constructed gas-admission element. The second process gas is fed to the gas-admission element from the gas-mixing system by means of a single pipeline. In the region of the gas-admission element, this feed line initially branches into two secondary feed lines. These two secondary feed lines then branch again, into in each case two tertiary feed lines, so that a total of four lines open out symmetrically into a mixing chamber. Individual passages, which branch further to a peripheral outlet opening, lead from this mixing chamber. As a result of fluctuations in the pipe diameter, this cascade-like splitting may lead to inhomogeneous flow conditions. Furthermore, this splitting is not suitable for single-sided, asymmetrical supply of two gases separately from one another.
The invention is based on the object of providing means which also allow the second process gas to be fed asymmetrically into the mixing chamber and with which, nevertheless, even with different kinds of process parameters or reactor geometries, it is possible to achieve a homogenous radial flow profile of the second process gas emerging from the outlet opening.
Furthermore, the invention is based on the object of providing a method with which a homogenous radial flow profile can be achieved by simple means.
The object is achieved by means of the invention specified in the claims. The method proposed in claim 1 provides that the second process gas flows through a flow-influencing element, in particular in the form of an annular throttle or a swirl generator, which is disposed downstream of the mixing chamber, and through an annular antechamber, which is disposed downstream of the flow-influencing element, and emerges through a gas-permeable gas-discharge ring.
The device described in claim 2 provides a flow-influencing element, for example an annular throttle or a swirl generator, which is disposed downstream of the mixing chamber, and an annular antechamber, which is disposed downstream of the flow-influencing element and is surrounded by a gas-permeable gas-discharge ring. The method parameters or the geometric parameters are selected in such a way that the flow resistance of the annular throttle is greater than the flow resistance of the gas-discharge ring by an extent which is such that the pressure difference between antechamber and mixing chamber is greater than the pressure difference between antechamber and process chamber. In a preferred configuration, these two pressure differences differ by at least a factor of 10. However, it is preferable for the difference to be higher. It is possible for a plurality of, in particular two, feed lines to open out asymmetrically into the mixing chamber. It is preferable for the feed lines to open out obliquely, in particular obliquely in the peripheral direction, into the mixing chamber. The gas streams which flow through the feed lines can be regulated individually. In particular, it is provided that different gases flow through the feed lines. By way of example, trimethylgallium or trimethylindium or the like can flow through one feed line. A metal-organic dopant can flow through the other feed line which opens out into the mixing chamber. The flow parameters are set in such a way that the lines all the way up to the outlet openings are held at a temperature at which decomposition of the reaction gases inside the gas-admission element is as far as possible avoided, so that in particular deposition on the gas-discharge ring is avoided. The annular throttle may consist of a gas-impermeable material which has a multiplicity of individual passages. In particular, the annular throttle may consist of quartz. The number of individual passages may be prime relative to the number of feed lines. The result of this is that different spacing in the peripheral direction is formed between the openings of the feed lines and of the individual passages. In particular, neither the points where the feed lines open out nor the individual passages are disposed point-symmetrically with respect to the central axis of the gas-admission element. This too contributes to homogenizing the radial flow profile. In a variant of the invention, the throttle consists of a porous material, for example of porous quartz. It is configured in particular as a frit. However, it may also consist of stainless steel, in particular stainless steel foam. The gas-discharge ring may likewise consist of a porous material, for example quartz. In a variant, the gas-discharge ring has a multiplicity of outlet passages, preferably a number of outlet passages which is prime relative to the individual passages of the annular throttle. The outlet passages may be formed by comb-like, in particular oblique incisions. The diameter of the gas-discharge ring may be greater than the diameter of the annular throttle. In the space of the mixing chamber in front of the annular throttle, a gas pressure which is considerably higher than the gas pressure behind the annular throttle is formed, so that a peripherally symmetrical gas stream flows through the annular throttle. This peripherally symmetrical gas stream opens out into the antechamber. Like the mixing chamber, the antechamber is disposed in the form of a ring around the central line. The antechamber is preferably undivided, so that a pressure which is virtually constant in the peripheral direction is established in the antechamber, this pressure being slightly higher than the pressure in the process chamber surrounding the gas-discharge ring. On account of this slight pressure difference, the gas flows homogeneously, preferably in a laminar flow, through the gas-discharge ring, specifically in the peripheral direction.
In a refinement of the invention, a swirl generator may also be disposed downstream of the mixing chamber. The gas which flows onwards out of the mixing chamber is passed through this swirl generator into a central annual flow chamber, where it becomes mixed on account of the swirling taking place at that location. This swirl generator may be followed by an annular throttle. According to a variant according to the invention, the device has a swirl generator, which is disposed downstream of the mixing chamber, and an annular antechamber, which is disposed downstream of the swirl generator and is surrounded by a gas-permeable gas-discharge ring. In the annular flow chamber which is disposed immediately downstream of the individual passages of the swirl generator, this swirl generator generates an annular flow, so that the reaction gases can mix there. It is preferable for the individual passages of the swirl generator, through which medium flows radially inward, to open out, with an inclination with respect to the axis transverse plane, into the annular flow chamber. The annular flow chamber can widen out in the downstream direction. On the radially outer side, it may be delimited by a diverter collar over which medium can flow in the upstream direction. On account of the flow deflection in the opposite direction, a steady toroidal swirl forms in the widened extension of the annular flow chamber. This swirl too effects homogenization of the gas composition. Together with the peripheral flow flowing around the central feed line in the annular flow chamber, this toroidal swirl effects an improvement in the intimate mixing of the gases. Since the volume of the annular flow chamber including the widening disposed downstream is in the region of one or several milliliters, the swirls have only a negligible storage function. An annular throttle may be associated with the diverter collar to the rear. This annular throttle may have a multiplicity of bores through which the gas flows. The bores may be directed at a deflector wall of a deflector collar. This deflector collar may be formed by a radially inwardly directed projection around which the gas flows.
A variant of the method consists in at least two feed lines opening out into the mixing chamber. According to the invention, more than ten times as much gas flows through one of the feed lines as through the at least one further feed line. This considerable difference in the volumetric flows results in improved intimate mixing of the gas in the mixing chamber. With this procedure, it is even possible to dispense with an annular throttle or a swirl generator. It is preferable for more than fifteen times as much gas to flow through the one feed line as through the at least one further feed line or through all the further feed lines together.